1. Technical Field
The present invention generally relates to evaporative emission control systems for automotive vehicles and, more particularly, to a method of controlling the proportional purge solenoid of an evaporative emission control system of an automotive vehicle.
2. Discussion
Automotive vehicles typically include a fuel tank for storing fuel and an evaporative emission control system for collecting volatile fuel vapors generated in the fuel tank. The evaporative emission control system includes a vapor collection canister, usually containing an activated charcoal mixture, to collect and store the fuel vapors. Normally, the vapor collection canister collects fuel vapors which accumulate during re-fueling of the automotive vehicle or from increases in fuel temperature. However, when conditions are conducive to purging the fuel vapors from the collection canister, a purge valve between an intake manifold of the vehicle's engine and the canister is opened by an amount determined by the engine control unit to purge the canister. Thereafter, the stored vapors are drawn into the intake manifold from the canister for ultimate combustion within a combustion chamber of the engine.
The amount the purge valve is opened is controlled by the amount of current delivered thereto. At low intake air flow levels and intake vacuum, the amount the purge valve should be opened may require a current to be delivered which is below a minimum threshold current level. As such, the purge valve receives either no current such that it remains closed when optimally it would be open for purging, or receives the minimum allowable current such that the purge valve is open more than an optimum amount. In either case, less than optimum control of the purge vapor flow through the purge valve results. In view of the foregoing, it would be desirable to provide an improved method of controlling the low-end flow characteristics of a purge valve such that enhanced control of purge flow rates through the purge valve at low intake air flow levels is provided.